Trump fired Comey for several reasons, which we need not go into. He reflected in an NBC interview "And in fact when I decided to just do it, I said to myself, I said 'you know, this Russia thing with Trump and Russia is a made-up story, it's an excuse by the Democrats for having lost an election that they should have won'."
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https://www.lewrockwell.com/lrc-blog/political-lynching-donald-trump/